2. Fiber Metal laminate according to claim 1, whereby the load factor
Klf is chosen such that 1.5.ltoreq.Klf≦2.5.

3. (canceled)

4. Fiber-metal laminate according to claim 1, wherein the strain
concentration factor Ksf is chosen such that 3.0<ksf<5.0
is satisfied.

5. Fiber-metal laminate according to claim 1, wherein the stiffness
factor kstiff is chosen such that kstiff≧1.34 is
satisfied.

6. (canceled)

7. Fiber-metal laminate according to claim 1, wherein a fraction of
fibers that satisfy the relationships (1) to (6) is at least 25% by
volume of the total volume of the fiber-reinforced composite layers.

8. Fiber-metal laminate according to claim 1, wherein the fraction of
metal MVF that satisfies the relationships (1) to (6) is >48% by
volume of the total volume of the metal layers.

9-12. (canceled)

13. Fiber-metal laminate according to claim 1, wherein the fiber and
metal properties for all fiber-reinforced composite layers and metal
sheets satisfy the relationships (1) to (6).

14-15. (canceled)

16. Fiber-metal laminate according to claim 1, wherein at least one of
the metal layers comprises an aluminum alloy with a stiffness of Et
metal>70 GPa.

24. Assembly of a fiber-metal laminate according to claim 1 and further
comprising a further element, the further element being bonded to the
fiber-metal laminate by a bonding layer comprising an adhesive and/or a
fiber-reinforced composite, or being connected by mechanical fastening
means.

25. (canceled)

26. Assembly according to claim 24, wherein the further element comprises
a flat or tapered plate from a metal selected comprising an aluminum
alloy, titanium alloy or steel alloy.

27. (canceled)

28. An aircraft structural primary part comprising at least one location
a fiber-metal laminate according to claim 1.

29. Part according to claim 28, comprising at least one aluminium lithium
sheet.

30-31. (canceled)

32. Fiber-metal laminate according to claim 7, wherein a fraction of
fibers that satisfy the relationships (1) to (6) is at least 30% by
volume of the total volume of the fiber-reinforced composite layers.

33. Fiber-metal laminate according to claim 7, wherein a fraction of
fibers that satisfy the relationships (1) to (6) is at
0.35<Vf<0.6.

34. Fiber-metal laminate according to claim 8, wherein the fraction of
metal MVF that satisfies the relationships (1) to (6) is >52% by
volume of the total volume.

35. Fiber-metal laminate according to claim 8, wherein the fraction of
metal MVF that satisfies the relationships (1) to (6) is >58% by
volume of the total volume.

36. Fiber-metal laminate according to claim 16, wherein at least one of
the metal layers comprises an aluminum alloy with a stiffness of Et
metal>75 GPa.

[0002] The behavior of engineering structures under load is determined by
many design parameters, and defining the optimum material for a specific
application is often a tedious task and moreover has to deal with
conflicting requirements. Among the commonly used engineering materials
are metals, like steel alloys, titanium alloys, aluminum alloys;
fiber-reinforced composites, like glass fiber composites, carbon fiber
composites, and aramid composites; and hybrid materials, further defined
below.

[0003] Fiber-reinforced composites offer considerable weight advantage
over other preferred materials, such as metals. Generally, the weight
savings are obtained at the sacrifice of other important material
properties such as ductility, toughness, bearing strength, conductivity
and cold forming capability. To overcome these deficiencies, new hybrid
materials called fiber-metal laminates have been developed to combine the
best attributes of metal and composites.

[0004] Fiber-metal laminates, such as those described in U.S. Pat. No.
4,500,589 and U.S. Pat. No. 5,039,571 are obtained by stacking
alternating thin layers of metal (most preferably aluminum) and
fiber-reinforced prepregs, and curing the stack under heat and pressure.
These materials are increasingly used in industries such as the
transportation industry, for example in ships, cars, trains, aircraft and
spacecraft. They can be used as sheets and/or a reinforcing element
and/or as a stiffener for (body) structures of these transports, like for
aircraft for wings, fuselage and tail panels and/or other skin panels and
structural elements of aircraft.

[0006] Although fiber-metal laminates may provide improved resistance to
fatigue (in particular crack propagation) over metal alloys, like
aluminum alloys, their behavior in a structure is still open for
improvement. It would in particular be highly desirable if the right
metal and fibers could be identified in terms of their properties to
achieve the right overall performance of the fiber-metal laminate (also
referred to as FML), based on these constituents.

[0007] It is an object of the invention to provide a fiber-metal laminate
comprising mutually bonded fiber-reinforced composite layers and metal
sheets with an optimal structural response.

SUMMARY OF THE INVENTION

[0008] According to one aspect of the present invention there is provided
a fiber-metal laminate comprising mutually bonded fiber-reinforced
composite layers and metal sheets having a range of fiber and metal
properties that yield an optimal structural response.

[0009] In accordance with the present invention a fiber-metal laminate is
provided comprising mutually bonded fiber-reinforced composite layers and
metal sheets, wherein the fiber and metal properties in at least one
combination of a fiber-reinforced composite layer and an adjacent metal
sheet satisfy the following relationships simultaneously:

[0015] Laminates according to the invention use a fiber-metal combination
that satisfies the above relations (1) to (6). Such configurations are
readily obtained by first selecting a metal, determining its ultimate
tensile strength and tensile Young's modulus at room temperature and
calculating the minimum required elastic fiber strains (tension and
compression) and elastic fiber modulus with the aid of relations (1) to
(3), using the minimum values for the strain concentration factor
Ksf, the stiffness factor Kstiff and the maximum value for the
load factor Klf, as defined in relations (4) to (6). Any fiber with
an elastic strain exceeding the calculated strain values (eq. 1 and 3),
and with an elastic modulus exceeding the calculated modulus (eq. 2) will
provide a laminate with the desired performance in a structure that is
designed for complex loading situations, which can be loaded in
compression, and/or tensile and/or fatigue.

[0016] With an optimal fiber-metal laminate is meant a fiber-metal
laminate that has the right combination of stiffness, static strength and
fatigue resistance when used in a structure. The invention is based on
the insight that in structures, one important design parameter relates to
strain concentration, and not, as is common practice to stress
concentration. Selecting material properties on the basis of the strain
concentration factor Ksf is unique and an important step forward.

[0017] To obtain a robust fiber-metal laminate according to the invention,
it is important that the stiffness factor Kstiff is chosen properly.
It has turned out that a stiffness factor Kstiff≧1.28 yields
optimum structural performance. Two major drivers are important in this
respect, i.e. structural fatigue behavior and general stiffness
(predominantly for compression-buckling and aero-elastic performance).
Due to manufacture of products and higher applied loads the crack
initiation of the metal layers may start prematurely. By choosing
Kstiff≧1.28 it is assured that the fiber has sufficient
stiffness to support the metal layer and match adequately the stiffness
of the metal applied in the fiber-metal laminate. Preferred are
fiber-metal laminates wherein the stiffness factor Kstiff is chosen
such that Kstiff≧1.34 is satisfied, and more preferred such
that Kstiff≧1.42 is satisfied.

[0018] The strain concentration factor for tensile and fatigue load
dominated fiber-reinforced composite structures typically varies between
2.75≦Ksf<5.7, since composite structures typically have an
ultimate tensile design strain value of 0.4%<ε<0.5%, carbon
fibers applied in such structures have failure strains in the order of
εf=1.5% and in general the composite fibers are in tension
more or less elastic until failure. It has been found that by adopting
the strain concentration factor range according to the invention
(equation (4)) fiber-metal laminates are obtained that are resistant to
tensile and fatigue structural loadings, but also to compression
loadings. High performing structures are subjected to a significant
amount of different loading cases, like, tension, compression,
alternating load case (fatigue load cases) etc. The ultimate positive to
negative load cases of these structures range between
1.5≦Klf≦3.5, consequently the elastic compression
strain of the fiber need to meet eq. 3, taking into account the load
factor Klf (eq. 6). However, it has to be realized also that fibers
can have a stress strain behaviour like metals; i.e. an elastic and
(semi-)plastic behaviour. Research has shown that this phenomenon can
happen especially for fibers in compression. The ultimate elastic strain
is the strain at which the fibre stiffness drops very significantly. In
that respect it is similar to the yield strain or even better
proportional limit of metals. Since the invention is related to fiber
metal laminates for optimum structures it is accepted that common
compression tests for composite structures will give adequate elastic
compression strain. Preferred fiber-metal laminates are those wherein the
load factor Klf is chosen such that 1.5≦Klf≦2.5,
and more preferably such that 1.5≦Klf≦2.0.

[0019] The laminates according to the invention are hardly sensitivity to
compression after impact degradation (quite often even better than its
metal ingredient) and therefore can be designed with a substantial higher
strain to failure as for fiber composite structures. Furthermore,
combining the fibrous composite layers and the metal sheets with
properties that satisfy equations (1) to (6) results in a fiber-metal
laminate with a higher stiffness than the metal sheets above the
proportionality limit thereof, and consequently will have increased yield
strength. The effect of reduced stiffness of the metal in the plastic
range is minimized therefore.

[0020] In another embodiment of the invention, the fiber and metal
properties for all fiber-reinforced composite layers and metal sheets
satisfy the relationships (1) to (6).

[0021] In a preferred embodiment, a fiber-metal laminate is provided
wherein the strain concentration factor Ksf is chosen such that
3.0<Ksf<5.0 is satisfied. Such laminates are advantageously
used in tensile and fatigue dominated structures, in other words in
structures that are not or less compression critical. In another
preferred embodiment, a fiber-metal laminate is provided wherein the
stiffness factor Kstiff is chosen such that Kstiff≧1.34
is satisfied. Such laminates are more advantageously used in structures
more sensitive to compression-buckling, in other words in structures that
are less tensile and fatigue loading critical. In another preferred
embodiment, a fiber-metal laminate is provided wherein the stiffness
factor Kstiff is chosen such that Kstiff≧1.42 is
satisfied. Such laminates are more advantageously used in highly
compression-buckling dominated structures, in other words in structures
that are still tensile and fatigue loaded as well, but hardly sensitive
to it, i.e. hardly governed by these loadings.

[0022] According to another aspect of the invention, a fiber-metal
laminate is provided wherein the fraction of fibers that satisfy the
relationships (1) to (6) is at least 25% by volume of the total volume of
the fiber-reinforced composite layers, more preferred at least 30% by
volume, and most preferred at least 35% by volume.

[0023] Particular preferred fiber-metal laminates according to the
invention are characterized in that the volume fraction of fibers that
satisfy the relationships (1) to (6) is 0.35<Vf<0.6 and more
preferred 0.40<Vf<0.54.

[0024] According to another aspect of the invention, a fiber-metal
laminate is provided wherein the metal volume fraction MVF>48%, more
preferably MVF>52% and most preferably MVF>58%.

[0025] According to a further aspect of the invention a fiber-metal
laminate is provided comprising a number of n mutually bonded and
alternating fiber-reinforced composite layers and metal sheets. The
number of layers n in the fiber-metal laminate of the invention can vary
between wide limits by preferably ranges from 3 to 100, more preferably
from 3 to 50.

[0026] According to the invention, the fiber-metal laminates preferably
comprise metal sheets of a different metal. Preferred laminates comprise
metal sheets having a thickness that ranges between 0.08 mm and 25.0 mm,
and more preferably between 0.2 mm and 12.5 mm, and most preferably
between 0.4 and 4.0 mm, the end points of the indicated ranges not
included.

[0027] In preferred embodiment, the metal is selected from steel alloys,
aluminum alloys, and titanium alloys in particular. In another preferred
embodiment, a fiber-metal laminate is provided wherein at least one of
the metal layers comprises an aluminum alloy with a stiffness of Et
metal>70 GPa, more preferably>75 GPa.

[0028] In a further preferred embodiment of the invention, a fiber-metal
laminate is provided wherein the fiber-reinforced composite layers
comprise high stiffness glass fibers having a tensile modulus of
elasticity>92.5 GPa and more preferably>100 GPa, Copol fibers
(developed by Tejin) or carbon fibers. Particularly preferred carbon
fibers are T1000 and/or IM10 carbon fibers. However, due to potential
galvanic action, the combination of most of the aluminium alloys with
carbon fibers is not preferred, in case these materials are adjacent to
each other. This combination is only preferred if the aluminium alloys
and carbon are shielded from each other by an insulating layer, like a
glass layer or with coated carbon fibers. The combination of these
aluminium alloys and carbon is further only preferred in a non humid
and/or non corrosive and/or inert environment, which will prevent or
largely reduce the potential of galvanic corrosion. Such application is
for instance for space applications. However it should be noticed that
new grades of aluminium alloys (in particular aluminium-lithium alloys)
can have a galvanic neutral outside surface and therefore do not or
hardly corrode with carbon fibers. These alloys, like the Airware®
alloys from Constellium, in combination of stiff carbon fiber are
therefore part of the invention. Furthermore, it should be realized that
potential galvanic action will (almost) not occur between carbon fibers
and steel alloys and titanium alloys.

[0029] The invention also relates to the use of a fiber-metal laminate
according to the invention, i.e. satisfying relations (1) to (6), in a
non corrosive, non humid or inert environment, even when the metal and
the fiber are not galvanic neutral with respect to each other.

[0030] The fiber reinforced composite layers may comprise substantially
continuous fibers that extend mainly in one direction and/or may comprise
substantially continuous fibers that extend mainly in two perpendicular
directions, such as in a woven fabric or cross ply. For advanced
structures with complex loading and stiffness requirements the fiber
reinforced composite layers may comprise substantially continuous fibers
that extend mainly in the rolling direction of the metal, perpendicular
to the rolling direction and with an to the rolling direction, whereby
the directions will be symmetrical to the rolling direction.

[0031] According to still another aspect of the invention, a fiber-metal
laminate is provided wherein the number of fiber-reinforced composite
layers and/or metal sheets varies between cross-sections, and therefore
also the fiber-metal laminate's thickness. Such laminates can also have a
tapered thickness and offer additional design freedom.

[0032] In yet another aspect of the invention, an assembly of a
fiber-metal laminate according to the invention and a further element is
provided, the further element being connected to the fiber-metal laminate
by a bonding layer, comprising an adhesive and/or a fiber-reinforced
composite or by mechanical means like riveting and/or bolting. The
further element preferably comprises a structural element selected from a
flat or curved doubler; a stiffener, such as an section, Z-stringer, hat
stringer, C-stringer, Y-stringer; a spar(section), rib(section),
shear-cleat and/or frame(section) of an aircraft structure. The further
element preferably is made from a metal, such as an aluminum alloy,
titanium alloy and/or steel alloy; a fiber-reinforced composite material,
such as those based on carbon fibres, aramid fibres, glass fibres, PBO
fibres, co-polymer fibres; hybrid materials, such as ARALL®,
Glare®, CentrAl®; a fiber-metal laminate according to the
invention, and combinations thereof

[0033] The invention further relates to an aircraft structural primary
part, such as a fuselage, wing and/or tail plane, comprising in at least
one location a fiber-metal laminate according to the invention.
Particularly preferred is such a part comprising at least one aluminum
lithium sheet.

[0034] The invention also relates to a method for selecting the metal
sheet properties and the fiber properties in a fiber-metal laminate
comprising mutually bonded fiber-reinforced composite layers and metal
sheets in order to obtain optimum fatigue properties of the fiber-metal
laminate. The method comprises selecting the metal alloy and the fiber in
a combination of a fiber-reinforced composite layer and an adjacent metal
sheet, such that the fiber and metal properties satisfy the following
relationships simultaneously:

[0040] Further embodiments of the method refer to embodiments of the
fiber-metal laminate, as described above and below in the context of the
fiber-metal laminate according to the invention. A preferred embodiment
of the method for instance is one in which the fiber and metal properties
of all fiber-reinforced composite layers and metal sheets in the laminate
satisfy relationships (1) to (6).

BRIEF DESCRIPTION OF THE FIGURES & TABLES

[0041] FIG. 1--is a view in perspective of a fiber-metal laminate
according to an embodiment of the present invention;

[0042] FIG. 2--is a view in perspective of a fiber-metal laminate
according to another embodiment of the present invention;

[0043]FIG. 3--is a view in perspective of a fiber-metal laminate
according to another embodiment of the present invention;

[0044]FIG. 4--is a view in perspective of a fiber-metal laminate
according to another embodiment of the present invention;

[0045]FIG. 5--is a view in perspective of a fiber-metal laminate
according to another embodiment of the present invention;

[0046]FIG. 6--is a view in perspective of a fiber-metal laminate
according to another embodiment of the present invention;

[0047]FIG. 7--illustrates the relationship of tensile stress and strain
of a metal sheet, as used in the fiber-metal laminate of the present
invention;

[0048]FIG. 8--illustrates a relationship of tensile stress and strain of
a layer of fiber-reinforced composite, as used in the fiber-metal
laminate of the present invention;

[0049] FIG. 9--illustrates the relationship between the minimum required
elastic fiber strain and the ratio of tensile strength to tensile modulus
of the metal as used in a fiber-metal laminate according to the present
invention;

[0050] FIG. 10--illustrates a relationship between the minimum fiber
compression strain and the ratio of tensile strength to tensile modulus
of the metal as used in a fiber-metal laminate according to the present
invention;

[0051]FIG. 11--illustrates a relationship between the minimum fiber
tension modulus and the tensile modulus of the metal as used in a
fiber-metal laminate according to the present invention;

[0052] Table 1--illustrates the mechanical properties of typical metals as
can be used in the fiber-metal laminate according to the invention.

[0053] Table 2--illustrates the properties of typical fibers as can be
used in the fiber-metal laminate according to the invention.

[0054] Table 3--illustrates some fiber-metal laminates which are according
to the invention as well as fiber-metal laminates which are not according
to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0055] In the following description, reference is made to the accompanying
drawings, which form a part hereof, and which show, by way of
illustration, specific embodiments in which the invention may be
practiced. The present invention, however, may be practiced without the
specific details or with certain alternative equivalent methods to those
described herein.

[0056] The basis of the present invention is a unique arrangement of
fiber-reinforced composite layers and at least one metal sheet. In
accordance with the invention a fiber-metal laminate is provided
comprising fiber-reinforced composite layers and metal sheets, wherein
the fiber properties relate to the metal properties in a specific manner,
as given by equations (1) to (6). The fiber-reinforced composite layers
preferably comprise fibers pre-impregnated with adhesive (prepreg). The
system of prepreg layers and metal sheets is preferably processed under
heat and pressure to cure the adhesive and form a solid panel or
component.

[0057] It has been discovered by the inventor that fiber-metal laminates
with fiber properties according to equations (1) to (6) have better
structural properties, i.e. an improved strength, stiffness, fatigue
resistance and damage tolerance than fiber-metal laminates known from the
state of the art. The parameters used in equations (1) to (3) are defined
in FIGS. 7 and 8. A significant difference in behavior can be observed
between a metal as used in the metal sheets of a fiber-metal laminate,
and fibrous composites. Metals show an elastic--plastic behavior as shown
in FIG. 7, whereby the metal can be plastically deformed above a yield
stress until the ultimate tensile strength of the metal σtu is
reached at a relatively large strain to failure. In most cases this
strain to failure is larger than 4% and may be as large as 18%, of which
the largest part is located in the plastic domain of the metal. The
tensile Young's modulus of the metal Et metal is reduced
considerably in the plastic domain. In contrast herewith, most fibrous
composites, largely due to the fibers, typically show almost elastic
behavior up to failure. However, fibers can show similar stress strain
behaviour as metals as shown in FIG. 8 for a fiber. This phenomenon can
occur more often in the compressive range of the fiber. In case the fiber
has an elastic-plastic behavior the ultimate elastic strain
(εultimate elastic tension & εultimate elastic
compression) is shown in FIG. 8.

[0058] The fiber properties of equations (1) to (3) are determined
according to ASTM norms.

[0059] More specifically, the ultimate elastic strain in tension, as well
as the Young's modulus of the fiber in tension are determined on fiber
samples in accordance with ASTM D2101. The ultimate elastic strain in
compression is determined on unidirectional composites according to ASTM
D-695. The ultimate elastic strains will be determined with the strain at
the yield strength determined by the off-set as mentioned by the ASTM
methods. The associated strain is referred to in FIG. 8 as
εyield tension & εyield compression
respectively. The ultimate elastic strains, εultimate
elastic tension & εultimate elastic compression will be:

εultimate elastic tension=εyield
tension-ε.sub.off-set

εultimate elastic compression=εyield
compression-ε.sub.off-set

For metal the off-set is most often taken at ε.sub.off-set=0.2%.
Since composites most often have a very low strain to failure,
particularly in compression, the off-set should be taken less than the
0.2%, most preferably equal or less than 0.1%. The large difference in
mechanical behavior between metals and composite reinforcing fibers has a
significant effect on stress concentrations in real life structures. The
invention is based on the insight that it is important to match the
constituent material properties in view of allowable strain
concentrations, and not stress concentrations.

[0060] The ensuing relation between the required minimum elastic tension
strain of the fiber in a fiber-metal laminate of the present invention
and the properties of the metal used (equation (1)) is graphically
depicted in FIG. 9 for different values of the strain concentration
factor Ksf. Fiber-metal laminates according to the invention use
fibers with an ultimate elastic tension strain lying on and between the
lines Ksf=2.75 and Ksf=5.0. Although the optimal relations in
principle hold for any metal properties, a lower limit for the metal
properties is preferably set for practical reasons. A preferred lower
limit for the parameter σtu/Et is 0.003. Below a value of
0.003 the properties of the metal and fiber in de fiber-metal laminate
will be too low. Hatched lines have been used in FIG. 9 for this
non-preferred area.

[0061] The ensuing relation between the required minimum elastic
compression strain of the fiber in a fiber-metal laminate of the present
invention and the properties of the metal used (equation (3)) is
graphically depicted in FIG. 10 for different values of the strain
concentration factor Ksf and load factor Klf. Fiber-metal
laminates according to the invention use fibers with an ultimate elastic
compression strain lying on and between the lines Ksf=2.75 with
Klf=3.5 and Ksf=5.0 with Klf=1.5. Although the optimal
relations in principle hold for any metal properties, a lower limit for
the metal properties is preferably set for practical reasons. A preferred
lower limit for the parameter σtu/Et is 0.002. Below a
value of 0.002 the properties of the metal and fiber in de fiber-metal
laminate will be too low. Hatched lines have been used in FIG. 10 for
this non-preferred area.

[0062] The ensuing relation between the required minimum modulus of the
fiber in a fiber-metal laminate of the present invention and the modulus
of the metal used (equation (2)) is graphically depicted in FIG. 11 for a
value of the stiffness concentration factor Kstiff=1.28. Fiber-metal
laminates according to the invention use fibers with a tensile elastic
modulus lying on and right of the line. A non-preferred area has metal
modulus below 50 GPa, as shown by the hatched line in FIG. 11.

[0063] The fiber-reinforced composite layers in the fiber-metal laminates
according to the invention are light and strong and comprise reinforcing
fibers embedded in a polymer. The polymer may also act as a bonding means
between the various layers. Reinforcing fibers that are suitable for use
in the fiber-reinforced composite layers depend on the choice of metal in
the metal sheets (see equations (1) to (3)) but may include glass fibers,
carbon fibers, copolymer fibres and metal fibers and/or combinations of
the above fibers. Preferred fibers include reinforcing fibers with a
relatively high tensile strength and/or stiffness, of which class high
modulus fibers, such as ultra high stiff glass fibers, Co-polymer fibers
and carbon fibers, are particularly preferred. Preferred reinforcing
fibers include carbon fibers. Particularly preferred fiber-metal
laminates comprise fiber-reinforced composite layers comprising T1000
and/or IM10 carbon fibers.

[0064] Examples of suitable matrix materials for the reinforcing fibers
include but are not limited to thermoplastic polymers such as polyamides,
polyimides, polyethersulphones, polyetheretherketone, polyurethanes,
polyphenylene sulphides (PPS), polyamideimides, polycarbonate,
polyphenylene oxide blend (PPO), as well as mixtures and copolymers of
one or more of the above polymers. Suitable matrix materials also
comprise thermosetting polymers such as epoxies, unsaturated polyester
resins, melamine/formaldehyde resins, phenol/formaldehyde resins,
polyurethanes, of which thermosetting polymers epoxies are most
preferred. The fibrous composites typically comprise from 25% to 60% by
volume of fibers.

[0065] In the laminate according to the invention, the fiber-reinforced
composite layer preferably comprises substantially continuous fibers that
extend in multiple directions, like 0°, 90° and under
angles symmetrically with respect to the rolling direction of the metal,
more preferably in two almost orthogonal directions (for instance
cross-ply or isotropic woven fabrics). However it is more preferable for
the fiber-reinforced composite layer to comprise substantially continuous
fibers that mainly extend in one direction (so called UD material). It is
advantageous to use the fiber-reinforced composite layer in the form of a
pre-impregnated semi-finished product. Such a "prepreg" shows generally
good mechanical properties after curing thereof, among other reasons
because the fibers have already been wetted in advance by the matrix
polymer.

[0066] Fiber-metal laminates may be obtained by connecting a number of
metal sheets and fiber-reinforced composite layers to each other by means
of heating under pressure and subsequent cooling. The fiber-metal
laminates of the invention have good specific mechanical properties
(properties per unit of density). Metals that are particularly
appropriate to use include steel (alloys) and light metals, such as
aluminum alloys and in particular titanium alloys. Suitable aluminum
alloys are based on alloying elements such as copper, zinc, magnesium,
silicon, manganese, and lithium. Small quantities of chromium, titanium,
scandium, zirconium, lead, bismuth and nickel may also be added, as well
as iron. Suitable aluminum alloys include aluminum copper alloys (2xxx
series), aluminum magnesium alloys (5xxx series), aluminum silicon
magnesium alloys (6xxx series), aluminum zinc magnesium alloys (7xxx
series), aluminum lithium alloys (2xxx, 8xxx series), as well as aluminum
magnesium scandium alloys. Suitable titanium alloys include but are not
limited to alloys comprising Ti-15V-3Cr-3Al-3Sn, Ti-15Mo-3Al-3Nb,
Ti-3A1-8V-6Cr-4Zr-4Mo, Ti-13V-11Cr-3Al, Ti-6Al-4V and Ti-6Al-4V-2Sn. In
other respects, the invention is not restricted to laminates using these
metals, so that if desired other metals, for example steel or another
suitable structural metal can be used. The laminate of the invention may
also comprise metal sheets of different alloys.

[0067] Although applying thinner metal sheets per se leads to higher costs
and is therefore not naturally obvious, it turns out that applying them
in the laminate leads to an improvement in the properties of the
laminate. The laminate according to the invention is additionally
advantageous in that only a few metal sheets have to be applied in the
laminate to be sufficient to achieve these improved properties. The same
advantages are achieved if the thickness of the prepreg in the
fiber-reinforced composite layers in the laminate is less than 0.8 mm,
and preferably inclusive between 0.1 and 0.6 mm.

[0068] A fiber-metal laminate according to the invention will generally be
formed by a number of metal sheets and a number of fiber-reinforced
composite layers, with the proviso that the properties of the fibers used
in the fiber-reinforced composite layers satisfy equations (1) to (6).

[0069] The outer layers of the fiber-metal laminate may comprise metal
sheets and/or fiber-reinforced composite layers. The number of metal
layers may be varied over a large range and is at least one. In a
particularly preferred fiber-metal laminate, the number of metal layers
is two, three or four, between each of which fiber-reinforced composite
layers have preferably been applied. Depending on the intended use and
requirements set, the optimum number of metal sheets can easily be
determined by the person skilled in the art. The total number of metal
sheets will generally not exceed 40, although the invention is not
restricted to laminates with a maximum number of metal layers such as
this. According to the invention, the number of metal sheets is
preferably between 1 and 30, and more preferably between 1 and 10, with
the metal sheets preferably having a tensile ultimate strength of at
least 0.25 GPa.

[0070] To prevent the laminate from warping as a result of internal
tensions, the laminate according to the invention can be structured
symmetrically with respect to a plane through the center of the thickness
of the laminate.

[0071] Fiber-metal laminate configurations according to the invention are
readily obtained by arranging (alternating) layers of fiber-reinforced
composite, preferably using prepregs, and at least one metal sheet. The
fiber-metal laminates can be designed in many different arrangements.

[0072] With reference to FIG. 1, a fiber-metal laminate according to one
embodiment is shown, wherein the total number of layers is 3, and wherein
layer 1 and layer 3 comprise a metal layer and layer 2 a fibrous
composite layer. Alternatively, layer 1 and layer 3 comprise a fibrous
composite layer and layer 2 is a metal layer. Layer 1 and layer 3 can
comprise the same metal or may be a different kind of metal. The fibrous
composite layer(s) may contain fibers in multiple directions as well as
different kind of fibers for which at least one of the fiber types in at
least one of the fibrous composite layer(s) fulfills the requirements set
in equations (1) to (6) with respect to at least one of the metal layers.
It should be noticed that the outside dimensions of the layers 1 to 3 are
not necessarily the same. For instance layer 3 and layer 2 can have the
same dimension, whereby the dimensions of layer 1 are larger. This can,
for instance, be the case for a large metal sheet with a local
reinforcement (layer 2 as composite layer and layer 3 as metal layer).

[0073] With reference to FIG. 2, a fiber-metal laminate according to
another embodiment is shown, wherein the total number of layers is n, and
wherein layer 1 is a metal layer and layer 2 is a fibrous composite
layer, which will be alternating until layer n-1 and layer n.
Alternatively, layer 1 is a fibrous composite layer and layer 2 is a
metal layer, which will be alternating until layer n-1 and layer n. The
alternating metal layers can be of the same metal or be a different kind
of metal. Also, at least one of the alternating fibrous composite layers
may contain fibers in multiple directions as well as different kind of
fibers, for which at least one of the fiber types in at least one of the
fibrous composite layer fulfills the requirements set in equations (1) to
(6) with respect to the adjacent metal layer, which is farthest away from
the centerline of the laminate. In case the outer layer of the laminate
is a fibrous composite layer, this layer preferably needs to fulfill the
requirements set in equations (1) to (6) with respect to its adjacent
metal layer. It should be noticed that the outside dimensions of the
layers 1 to n are not necessarily the same.

[0074] With reference to FIG. 3, yet another embodiment of the fiber-metal
laminate according to the invention is shown. In the embodiment shown,
layer 1 and layer 3 are a metal layer and layer 2 is a fibrous composite
layer or, alternatively, layer 1 and layer 3 are a fibrous composite
layer and layer 2 is a metal layer. Layer 1 and 3 can be the same metal
or be a different kind of metal. The fibrous composite layer(s) may
contain fibers in multiple directions as well as different kind of fibers
for which at least one of the fiber types in at least one of the fibrous
composite layers fulfills the requirements set in equations (1) to (6).
Also layer 1, 2 and/or 3 can be a laminate according to FIG. 1 or 2 with
respect to at least one of the metal layers. It should be noticed that
the outside dimensions of the layers 1 to 3 are not necessarily the same.

[0075] With reference to FIG. 4, yet another embodiment of the fiber-metal
laminate according to the invention is shown. In this embodiment, layer 1
is a metal layer and layer 2 is a composite layer, which will be
alternating until layer n-1 and layer n or, alternatively, layer 1 is a
composite layer and layer 2 is a metal layer, which will be alternating
until layer n-1 and layer n. The alternating metal layers can be of the
same metal or be a different kind of metal, and at least one of the
alternating composite layers may contain fibers in multiple directions as
well as different kind of fibers, for which at least one of the fiber
types in one of the composite layer(s) fulfills the requirements set with
respect to the adjacent metal layer which is farthest away from the
centerline of the laminate. In case the outer layer of the laminate is a
fibrous composite layer, this layer needs to fulfill the requirements set
in equations (1) to (6) with respect to its adjacent metal layer. As
shown in FIG. 4, the number of fiber-reinforced composite layers and/or
metal sheets varies between cross-sections. Assuming the metal sheets are
the white layers, and the fibrous composite layers are the darker layers,
the number of fibrous composite layers varies from (n-1)/2 layers in a
cross-section at the left of the figure to zero in a cross-section at the
right of the figure (the fibrous composite layers are interrupted), which
results in a fiber-metal laminate with a varying, i.e. a tapered
thickness. Also layer 1, 2 and/or 3 can be a laminate according to FIG. 1
or 2 It should be noticed that the outside dimensions of the layers 1 to
n are not necessarily the same.

[0076] With reference to FIG. 5, still another embodiment of the
fiber-metal laminate according to the present invention is shown, in
which layer 1 and layer 3 are a metal layer and layer 2 is a fibrous
composite layer or, alternatively, layer 1 and layer 3 are a fibrous
composite layer and layer 2 is a metal layer, in which layer 1 and 3 can
be the same metal or be a different kind of metal, and in which the
fibrous composite layer(s) may contain fibers in multiple directions as
well as different kind of fibers for which at least one of the fiber
types in one of the fibrous composite layer(s) fulfills the requirements
set in equations (1) to (6). Also layer 1, 2 and/or 3 can be a laminate
according to FIG. 1 or 2

[0077] With reference to FIG. 6, yet another embodiment of the fiber-metal
laminate according to the present invention is shown, in which layer 1 is
a metal layer and layer 2 is a composite layer, which will be alternating
until layer n-1 and layer n. Alternatively, layer 1 is a composite layer
and layer 2 is a metal layer, which will be alternating until layer n-1
and layer n. The alternating metal layer can be of the same metal or be a
different kind of metal, and at least one of the alternating composite
layers may contain fibers in multiple directions as well as different
kind of fibers, for which at least one of the fiber types in one of the
composite layer(s) fulfills the requirements set in equations (1) to (6)
with respect to the adjacent metal layer, which is farthest away from the
centerline of the laminate. In case the outer layer of the laminate is a
fibrous composite layer, this layer needs to fulfill the requirements set
in equations (1) to (6) with respect to its adjacent metal layer. Also
layer 1, 2 and/or 3 can be a laminate according to FIG. 1 or 2

[0078] The laminates are produced by preparing a stack of fibrous
composite and metal sheets in the sequence as exemplified in FIGS. 1 to
6, for example on a flat or curved mold. After lamination, the overall
structure is cured at a temperature suitable for the matrix resin,
preferably an epoxy or thermoplastic resin, for instance in an autoclave,
and preferably under vacuum in order to expel entrapped air from the
laminate. For most applications, an epoxy or thermoplastic resin with a
high glass transition temperature will be most suitable. Any epoxy resin
may be used however. Epoxy resins are generally cured at or slightly
above room temperature, at a temperature of approximately 125° C.
or at a temperature of approximately 175° C. After curing under
pressure a consolidated laminate is obtained. As mentioned above, it is
also possible to use a thermoplastic resin.

Examples and Comparative Examples

[0079] The invention will be illustrated by several Examples, whereby the
properties of Table 1 & 2 have been used. The Examples are shown in Table
3, whereby Ksf=2.75, Kstiff=1.28 and Klf=3.5 is applied.

[0080] Table 3 clearly shows that known laminates like GLARE (aluminium
2024-T3 or 7475-T761 with S2-glass fibers), ARALL (aluminium 2024-T3 or
7475-T761 with original Kevlar fibers)and Ti-Gr (Ti-6Al-4V and T300
carbon fibres) do not fulfill the requirements set forth in this
invention. It also shows clearly that fiber metal laminates with new
fibers like Copol fibres in relation with aluminium alloy are according
to the invention.

[0081] The invention underlines the importance of fiber stiffness,
especially for laminates with glass fibers. Furthermore, it underlines
the importance of elastic compressive strain for almost all other fibers.

[0082] The fiber-metal laminate according to the present invention is
advantageously used in constructing load bearing structures, such as
aircraft structures. It is also advantageously used in an assembly with a
further element, the further element being bonded to the fiber-metal
laminate by a bonding layer, comprising an adhesive and/or a
fiber-reinforced composite. The further element may comprise an aircraft
(sub)structure, such as a flat or curved doubler; a stiffener, such as an
section, Z-stringer, hat stringer, C-stringer, Y-stringer; a
spar(section), rib(section), shear-cleat and/or frame(section).